Warn if LOCKTAG_TUPLE is held at commit, under debug_assertions.

The current use always releases this locktag.  A planned use will
continue that intent.  It will involve more areas of code, making unlock
omissions easier.  Warn under debug_assertions, like we do for various
resource leaks.  Back-patch to v12 (all supported versions), the plan
for the commit of the new use.

Reviewed by Heikki Linnakangas.

Discussion: https://postgr.es/m/20240512232923.aa.nmisch@google.com

src/backend/storage/lmgr/lock.c

@@ -2305,6 +2305,16 @@ LockReleaseAll(LOCKMETHODID lockmethodid, bool allLocks)
 				locallock->numLockOwners = 0;
 		}
 
+#ifdef USE_ASSERT_CHECKING
+
+		/*
+		 * Tuple locks are currently held only for short durations within a
+		 * transaction. Check that we didn't forget to release one.
+		 */
+		if (LOCALLOCK_LOCKTAG(*locallock) == LOCKTAG_TUPLE && !allLocks)
+			elog(WARNING, "tuple lock held at commit");
+#endif
+
 		/*
 		 * If the lock or proclock pointers are NULL, this lock was taken via
 		 * the relation fast-path (and is not known to have been transferred).